1. Field of the Invention
This invention relates to electromagnetic devices and, in particular, variable impedance devices, e.g. current limiting devices such as are used in power supply systems in order to control the current supplied from a source to a load. Such devices are particularly useful in limiting the current which flows under fault conditions.
2. Description of Related Art
European Patent Application No. 353449 discloses a current limiting device comprising a coil and an associated core of magnetically soft material such as iron forming a closed magnetic circuit, the coil being wound on a hollow cylinder of superconducting material which encircles the core and influences flux linkage between the coil and the core in dependence upon the magnitude of the current supplied to the coil.
Whilst such an arrangement presents a relatively low impedance to currents of normal magnitude and is effective in limiting current flow through the coil in response to the occurrence of large magnitude fault currents, it suffers from the drawback that the dimensions of the cylinder are determined by the dimensions of the coil which means that the larger the coil the larger the cylinder must be. The cylinder has to be cooled to a suitable temperature in order to secure the superconducting properties of the material and consequently the larger the cylinder the greater its surface area and the greater the cooling power necessary to maintain the cylinder at a desired temperature. In addition, fabrication of large diameter cylinders from superconducting materials is not a trivial matter.